In general, as an antifouling technique, it is desirable to impart oil repellency to make stains difficult to adhere and hydrophilicity to make the adhered stains be easily removed by washing with water to the substrate. As a technique to impart hydrophilicity to the substrate surface, a method in which a photocatalyst film is formed by immobilizing a photocatalyst such as titanium oxide on the substrate surface and the adhered stains are washed out by superhydrophilization by the action of the photocatalyst is known (Patent Document 1).
However, in a case where a photocatalyst film is used, although a function of easily removing stains can be obtained, there is a problem that the characteristic of preventing adhesion of stains is not sufficient. In particular, in an environment in which light necessary for exhibiting a photocatalytic function is not sufficiently obtained, sufficient antifouling properties are not obtained in some cases.
On the other hand, as a technique to impart oil repellency to the substrate surface, a method in which a fluorine-based compound is mainly used as a surface processing agent is known. As the fluorine-based compound, fluororesins such as polytetrafluoroethylene (PTFE) and compounds having a perfluoroalkyl group in the molecule are known. In a case where these fluorine-based compounds are used, although the water repellency is high, there is a problem that it is difficult to wipe off oil stains adhered to the surface or to remove by washing with water, like general hydrophobic coating films. In particular, in the case of being used in an environment where water is applied, inversely, there is a problem that oil stains easily adhere thereto.
A fluororesin hydrophilized by subjecting the surface to a plasma surface treatment, a flame treatment, an ozone treatment, or the like has been proposed (Patent Document 2), but the above-described special treatments are necessary, and the obtained hydrophilicity is also not sufficient. In addition, if hydrophilicity is imparted, there is a problem that the function of oil repellency is not obtained. Furthermore, there is a problem that a fluororesin is difficult to apply to a substrate surface and processed.
From the above, to impart a sufficient antifouling function to an object to be treated such as a substrate, a hydrophilic oil repellent which exhibits excellent hydrophilicity and excellent oil repellency at the same time has been demanded. In addition to the antifouling function, an excellent hydrophilic oil repellent is useful in a wide range of applications requiring quick drying properties of water accompanying improvement of wettability, an antifogging property, and oil-water separability. Among these, the hydrophilic oil repellent is particularly useful in oil-water separability applications.
However, among compounds in the related art, there is no compound which exhibits excellent hydrophilicity and excellent oil repellency at the same time, and it is difficult to impart a sufficient antifouling function and an oil-water separating function thereto.